A computational study of CuCrX2 (X = S, Se, Te) for intermediate band solar cell: Conceptual density functional theory approach

In conventional semiconducting materials, charge carriers i.e. electrons are readily stimulated to the conduction band (CB) from the valence band (VB). Three-stage photon conversions can be established by adding a partially filled intermediate band (IB) into the forbidden energy gap. In this technique, electrons can be stimulated from VB to IB, from IB to CB, and directly also from VB to CB. This results in a larger photocurrent without a trade-off in photovoltage for the Intermediate-band Solar Cell (IBSC) [1]. The efficacy of IBSC is reported as 63.2% which is a higher percentage than that of a single-junction photovoltaic cell (40.7%) [2]. It is reported that the performance of IBSC can be increased by up to 80% by introducing more IBs [3]. IBSC has gained a considerable amount of consideration lately, due to its strong potential for increased power conversion efficiency. Despite being popular in the market, there is still an urgent need for a promising cost-effective solution to the traditionally used single-junction cells. IBSC is one such promising candidate which is a combination of different semiconductors. It basically means stacking materials with a different bandgap that is capable of absorbing a wide range of the solar spectrum. The working mechanism of IBSC is shown in Figure-1. The concept of multi-junction solar cells has the tendency of not just improving efficiency but also reducing the energy losses produced by optically excited carriers due to thermal relaxation.

Ternary semiconductors I-III-VI2 (Idouble bondCu, Ag; IIIdouble bondAl, Ga, In; VIdouble bondS, Se, Te) have been proven as potential substitutes for Pb- and Cd-chalcogenide-based materials because of their intrinsic minimal toxicity. These materials exhibit broad light absorption controllability, comprising a spectrum that varies from 47 nm to 1200 nm, based on the size and structure of crystals [[4], [5], [6], [7], [8]]. Additionally, it also possesses notably significant global Stokes shifts and optoelectronic properties [[4], [5], [6], [7]]. Semiconducting materials I-III-VI2 are found suitable for quantum dot-sensitized photovoltaic devices, light-emitting diodes, and life sciences applications [4,6,[9], [10], [11], [12]]. These systems exhibit potential as photosensitizers, for instance, for the conversion of CO2 [13] or H2 generation [14]. Furthermore, high global Stokes shifts and controllable emission and absorption spectrum enable these compounds to optimum luminophores for fluorescent photovoltaic cells [4,[15], [16], [17], [18]]. There have been extensive studies reported on chalcopyrite and kesterite-type structures in recent years [[19], [20], [21], [22]]. Cu(In, Ga)Se2 and Cu2Zn(S, Se)4 being the popular structures have shown an efficiency of 22.3% and 12.6% respectively [21,22]. But according to Shockley and Queisser's thermodynamic analysis efficiency of single bandgap photovoltaic cells for energy conversion is limited to 31.0% if considered at Sun concentration 1 [23]. To overcome this issue of broad spectral absorption, IBSC was introduced in the year 1977 [2,5,[24], [25], [26]]. It is predicted that IBSC efficiency could reach up to 46.8% at 1Sun while at the full concentration, it is estimated to be 63.2% [7]. However, the ideal bandgap for IBSCs at 1Sun and full concentration is 2.40 eV and 1.93 eV respectively [27,28]. Density Functional Theory (DFT) has been widely used over a wide range of subjects and has been found quite successful in many fields. It has been found useful in studying the important parameters for newly synthesized various eco-friendly compounds and other drugs' properties [[29], [30], [31], [32]]. It is successfully applied to doped semiconducting materials that possess IB properties in recent years [ 1, 5, 7, 25, 26]. Most fascinatingly, an IB compound with a vanadium-doped crystal structure, In2S3 was established practically on the basis of DFT results [1,27,28]. The empirical IB absorbance characteristic was completely in concurrence with the DFT data. The large energy gap of semiconducting material CuGaS2 (Eg = 2.4 eV) is inappropriate to be utilized as a light-absorbing layer in single-junction photovoltaic modules. Though, it provides an energy gap that is essentially adequate and might be utilized as a perfect composite for IBSC [1,28]. CuGaS2, which is structurally equivalent to Cu(In, Ga)Se2 has significant gain from a commercialization standpoint when it comes to utilizing CIGS-linked advancements for the maximum efficacy photovoltaic devices [1,33]. Thus, CuGaS2 is a viable candidate for IBSC. According to DFT results, the IB could be generated by depositing titanium or chromium at the gallium position of CuGaS2 [ 1, 14, 27]. Chen et al. [1] outlined a theoretical and experimental study of CuGa1-xCrxS2. The experimental findings indicate three-photon absorbance in CuGa1-xCrxS2. The authors also stated that broad solar absorption owing to the presence of an intermediate layer is essential for solar cells. Palacios and his team studied titanium and chromium-doped semiconductor material CuGaS2 and found that the replacement of gallium atom with titanium or chromium results in an IB material which may be utilized to develop new and more effective solar cells [[34], [35], [36], [37]]. It is reported that the introduction of titanium and chromium atoms in CuGaS2 emits substantial charge carriers and large open-circuit voltage [1,38,39]. Recently, titanium and iron-doped chalcopyrite materials are investigated for their probable applications in photovoltaic devices [[40], [41], [42], [43], [44]]. Korotaev et al. [18] reported electrical resistivity, heat capacity, and Seeback coefficient of CuCrS2. Wang et al. [45] investigated chromium-doped p-CuGaS2 materials experimentally. The authors stated that the conductivity of chromium-doped CuGaS2 material increases with an increase in chromium doping concentration, similarly the carrier density and mobility also increase. The authors also reported that IB is responsible for the rise in photocatalytic carriers.

In this work, we have studied CuCrX2 (X = S, Se, Te) with the usage of DFT methodology. Different exchange-correlation like B3LYP, CAM-B3LYP, ωB97XD, MPW1PW91, HSEH1PBE, PBEPBE, and TPSSTPSS with LANL2DZ basis set is utilized for computing the DFT-based global descriptors. Parameters including electronic, thermo-chemical, and optical properties of these systems are also analyzed and discussed.

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